A typical surgical tourniquet system of the prior art includes a tourniquet cuff for encircling a patient's limb at a desired location, a tourniquet instrument, and flexible tubing connecting the cuff to the instrument. In some surgical tourniquet systems of the prior art, the tourniquet cuff includes an inflatable portion, and the inflatable portion of the cuff is connected pneumatically through one or two cuff ports by flexible plastic tubing to a tourniquet instrument that includes a pressure regulator to maintain the pressure in the inflatable portion of the cuff, when applied to a patient's limb at a desired location, at a pressure above a minimum pressure required to stop arterial blood flow past the cuff during a time period suitably long for the performance of a surgical procedure. Many types of such pneumatic surgical tourniquet systems have been described in the prior art, such as those described by McEwen in U.S. Pat. No. 4,469,099, U.S. Pat. No. 4,479,494, U.S. Pat. No. 5,439,477 and by McEwen and Jameson in U.S. Pat. No. 5,556,415 and U.S. Pat. No. 5,855,589. Some additional representative dual-port tourniquet cuffs of the prior art, suitable for use as elements of dual-port surgical tourniquet systems, are described in U.S. Pat. No. 4,635,635, U.S. Pat. No. 5,454,831, U.S. Pat. No. 5,439,477, U.S. Pat. No. 5,741,295 and U.S. Pat. No. 5,649,954.
To achieve better overall safety and performance, and in particular to achieve greater speed and accuracy in controlling the pressure in the tourniquet cuff, some advanced tourniquet systems include tourniquet cuffs that have two separate pneumatic cuff ports, so that two separate pneumatic passageways can be established between the inflatable portion of the cuff and the tourniquet instrument, by separately connecting flexible plastic tubing between each port and the instrument. Such systems are often called dual-port tourniquet systems. In one such dual-port tourniquet system of the prior art, described in U.S. Pat. No. 4,469,099, the pneumatic pressure regulation elements within the tourniquet instrument communicate pneumatically with the inflatable portion of the cuff through one port, and a pressure sensor within the tourniquet instrument communicates pneumatically with the inflatable portion of the cuff through the second port. This configuration enables more accurate sensing, monitoring and regulation of the actual pressure in the inflatable portion of the cuff that encircles the patient's limb, in comparison to single-port tourniquet systems.
In a typical single-port tourniquet system of the prior art, the tourniquet cuff has only one port and only one pneumatic passageway is established between the tourniquet cuff and the instrument. The cuff pressure must be sensed indirectly, through the same tubing and port that is used to increase, decrease and regulate the pressure in the cuff during surgery. As a result, in such a single-port tourniquet system of the prior art, the accuracy and speed of pressure regulation, and the accuracy of the sensed cuff pressure, are affected by the pneumatic flow resistance within the single port and within the flexible plastic tubing that pneumatically connects the port and cuff to the tourniquet instrument.
The increased accuracy possible with dual-port surgical tourniquet systems of the prior art is important for safety in surgery because of the well recognized importance of regulating the actual pressure in the tourniquet cuff near the minimum pressure that will reliably stop blood flow into the limb distal to the cuff during surgery: Higher tourniquet pressures are associated in the medical literature with higher risks of injury to the nerves and soft tissue of the patient's limb and other hazards, and lower tourniquet pressures may allow the flow of arterial blood past the cuff and into the limb and surgical field, thus complicating the surgery, extending the duration of surgery, and leading to congestion of the limb and other hazards.
The increased accuracy of pressure regulation that is possible with dual-port tourniquet systems facilitates increased accuracy in the determination of the minimum effective tourniquet pressure needed for a specific patient and surgical procedure as described by McEwen in U.S. Pat. No. 5,439,477. The greater accuracy of pressure regulation possible with dual-port tourniquet systems also allows such systems to respond more rapidly, more accurately and more safely to changes associated with movement of the patient's limb and cuff during surgery, and to changes associated with switching between cuffs for bilateral limb surgery, or for intravenous regional anesthesia. Dual-port tourniquet systems also facilitate rapid, accurate and safe adaptation to certain physiologic changes of the patient during surgery, as described for example in U.S. Pat. No. 4,479,494, and facilitate faster, more accurate and safer responses to changes in effective venous pressure in the limb distal to the cuff associated with the injection of anesthetic agents for intravenous regional anesthesia, as described for example in U.S. Pat. No. 5,254,087.
One hazard associated with all pneumatic surgical tourniquet systems is the possible occlusion, or blockage, of the pneumatic passageway between tourniquet instrument and cuff, or within the cuff itself. In many surgical procedures, the pressure regulator is located remotely from the patient, necessitating the use of long and flexible plastic tubing extending from the instrument and around and between equipment and staff to the cuff applied to the patient. Also, the ports of sterile disposable tourniquet cuffs are often long and soft and flexible, to allow the ports to be easily positioned away from the sterile surgical field, to minimize obstruction of the surgical field, and to allow the connection of the sterile ports to non-sterile tubing to be done outside of the surgical field. As a result, it is possible that the tubing or the port may become occluded for example by bending, kinking or pinching.
Also, it is recognized in the prior art that occlusions may occur within the inflatable portion of tourniquet cuff itself, thereby preventing the tourniquet cuff from effectively and uniformly applying pressure to the limb as intended. Some tourniquet cuffs of the prior art have specifically recognized and addressed this hazard in the design and manufacture of the inflatable portion of the cuffs and their associated ports. In general, if any occlusions occur in the tubing, cuff ports or inflatable portion of the cuff and are not promptly detected and remedied, the result may be hazardous for the patient.
The occlusion hazard is most apparent in certain single-port tourniquet systems of the prior art, in which any occlusion of the single port or tubing between the instrument and tourniquet cuff may prevent the tourniquet instrument from sensing and regulating the pressure in the cuff. Thus the actual pressure in the cuff may decrease substantially below the desired tourniquet pressure to a level where the cuff is completely depressurized, or the actual pressure may increase substantially above the desired tourniquet pressure, without any indication to the surgical staff and without any audio-visual alarms being activated to alert the staff. In effect, the monitoring and regulation of cuff pressure from the tourniquet instrument stops at the location of the occlusion. Such low or high pressures in the tourniquet cuff may be hazardous to the patient, for reasons indicated above.
Apparatus for detecting occlusions in both single-port and dual-port surgical tourniquet systems has been described in the prior art by McEwen and Jameson in U.S. Pat. No. 5,681,339 and U.S. Pat. No. 5,935,146. The apparatus and related method are based on an adaptation of the principle of acoustic reflectometry for pneumatic tourniquets: A brief pneumatic pressure pulse is periodically introduced from the tourniquet instrument into the pneumatic tubing connected to the cuff, and the instrument then analyzes reflections arising from pressure pulse which occur when the pulse encounters a change in the cross-sectional area of the pneumatic passageway as the pulse propagates along the passageway.
The apparatus and method of McEwen and Jameson described in U.S. Pat. No. 5,681,339 and U.S. Pat. No. 5,935,146 has been implemented in some commercially available single-port tourniquet systems of the prior art. However, there are a number of significant limitations. First, for implementation, additional components and higher resolution components must be included within such single-port tourniquet instruments, resulting in additional cost. Second, to be effective, the apparatus and method of McEwen and Jameson requires that the flexible plastic tubing between the tourniquet instrument and the cuff port have a fixed tubing length and a constant cross-sectional area, and requires that the cuff port connecting the tubing to the inflatable portion of the cuff also have a similar cross-sectional area to the flexible tubing that connects the instrument to the cuff port. Both of these requirements limit the general utility of the apparatus and method because in general it would be more useful to allow occlusion detection in tubing and in cuff ports having different lengths, diameters and cross-sectional areas. Another limitation of the apparatus and method of McEwen and Jameson is that it cannot reliably detect occlusions within the inflatable portion of a cuff, and it would be desirable to have an occlusion detector that could do so, because such occlusions within the cuff can occur in practice. A related limitation is that the apparatus and method may erroneously indicate occlusions if very small cuffs such as pediatric tourniquet cuffs are used, especially if such cuffs are wrapped tightly around a patient's limb. In such cases, the effective inflation volume of the cuff is very small, and as a result an occlusion at the cuff may erroneously be indicated. Still another limitation is that, to be effective, the apparatus and method of McEwen and Jameson requires known and fixed specifications for any pneumatic valves inserted between the tourniquet instrument and cuff. In practice this requirement can present a problem because additional pneumatic valves having unknown specifications may be inserted by surgical staff between the cuff and instrument to facilitate bilateral limb surgery and intravenous regional anesthesia (Bier block anesthesia).
The apparatus and method described by McEwen and Jameson in U.S. Pat. No. 5,681,339 and U.S. Pat. No. 5,935,146 have not been implemented in any commercial dual-port tourniquet systems of the prior art, primarily because of increased complexity and cost. The additional cost of implementation in a dual-port system would be approximately double the cost of implementation in a single-port system, because each tube and connected port must be separately monitored for occlusions.
In the prior art, an apparatus specifically intended for detecting occlusions in some dual-port surgical tourniquet systems has been described by Birmingham and Manes in U.S. Pat. No. 4,520,819. The apparatus employs a pair of pressure differential switches in the instrument, connected anti-parallel between the pneumatic passageways connecting the instrument to the dual-port cuff, which activate an occlusion alarm when a pressure differential develops across them. The underlying assumption is that a pneumatic occlusion will eventually result in a pressure differential that is sufficiently large to be detected within the instrument. However, inherent limitations in such apparatus for detecting occlusions have prevented it from being incorporated into commercial dual-port tourniquet systems of the prior art.
A basic limitation is that the apparatus of Birmingham et al. cannot detect an occlusion that is not associated with a pressure differential. Significantly, it cannot detect a simultaneous kinking, pinching and blockage of both tubes connecting dual-port cuffs to the instrument, as can occur during surgery. In such a case, there may be no pressure differential in the passageways between the instrument and the kink or blockage, and thus no alarm. In such a circumstance, the dual-port cuff could be completely disconnected from the tubing without an occlusion alarm being activated.
Another important limitation of the apparatus of Birmingham et al. is that inherent flow resistance within the flexible plastic tubing, ports and cuff may produce a significant pressure differential during inflation or deflation of the cuff, thus activating false occlusion alarms. A related limitation is that, if an effort is made to avoid such false alarms by raising the level of pressure differential at which an occlusion alarm is activated, then the detection of an actual occlusion may be delayed or prevented, leading to a hazardously high or low pressure in the tourniquet cuff applied to the patient. Yet another limitation of the apparatus of Birmingham et al. is that it requires the inclusion of additional electronic components and pneumatic connections within the dual-port tourniquet instrument, increasing costs and introducing potential secondary hazards associated with malfunctions of such components that may be difficult to detect.
There is a need for an improved occlusion detector for dual-port surgical tourniquet systems that overcomes the above-described problems and limitations of occlusion detectors of the prior art. Ideally, an occlusion detector would not require the addition of special components, circuitry or pneumatic connections within a dual-port tourniquet instrument, and thus would not increase instrument costs and would not introduce secondary hazards associated with potential malfunctions of such additional components, circuitry and pneumatic connections. An occlusion detector should promptly and reliably detect occlusions in the tubing, in the cuff ports, and within the inflatable portion of the dual-port cuff itself, regardless of whether the cuff is sterile or reusable, regardless of the size, shape, composition and length of the cuff ports, and regardless of the type of cuff port connectors. Also, an occlusion detector should reliably detect occlusions within inflatable portions of dual-port tourniquet cuffs that have a wide variety of shapes, compositions and sizes, and thus a wide variety of inflatable cuff volumes when applied to a limb.
An occlusion detector for dual-port tourniquet systems should detect occlusions in only one tube or connected port, and should also detect simultaneous occlusions of both cuff ports or both connected tubes. To be generally useful, an occlusion detector for dual-port tourniquet systems should not require that the lengths, sizes and shapes of tubing be known in advance for reliable operation, nor should it unduly limit the types of pneumatic valves that may be inserted between cuff and instrument to facilitate bilateral limb surgery and intravenous regional anesthesia. The present invention addresses the need for an improved occlusion detector for dual-port surgical tourniquet systems.